1. Field of the Invention
This invention relates to improvements of coating type magnetic recording mediums, particularly, relates to the magnetic recording mediums suitable for high density digital signal recording without degradation of electromagnetic conversion characteristics and running durability even when the thickness of the magnetic recording medium is reduced.
2. Description of the Related Arts
Recently, as the demand for high density recording increases, the recorded signal wavelength in the magnetic recording is becoming shorter.
Responding to the demand for the high density recording, there has been proposed a coating type magnetic recording medium employing ferromagnetic metal powder and a deposition type magnetic recording medium employing a metal thin layer as a magnetic layer. Between the two types, the coating type magnetic recording medium is more excellent in productivity and has erosion resistance to the oxidation of the magnetic layer.
As to the high density recording, there is a problem regarding the thickness of the magnetic recording medium.
An excessive thickness of the recording medium causes a reduction of an output level in high frequency signals (short wavelength signals) due to a self-demagnetization loss in recording and a thickness loss in reproduction, thus, an overwriting characteristic is also degraded. Accordingly, the recording medium used for the high density digital recording is required to have a small thickness of the. recording layer. In order to provide a thin recording layer, there is proposed a multilayer recording medium having a non-magnetic layer as an underlayer provided under the recording layer.
Further, in order to satisfy the demands for a long playing time of the recording medium along with demands for the size reduction of the apparatus, attempts to reduce an overall thickness and a width of the recording medium are being made.
However, the reduction of the thickness of the recording medium reduces the stiffness of the recording medium thus degrades a head-medium (for instance, a head-tape) contact characteristic, resulting in degradation of the electromagnetic conversion characteristics.
Therefore, there have been proposed a several counter-measures to improve the head-tape contact characteristic.
Exemplarily, in the Japanese Patent Laid-Open Publication S63-191315/1988, there is proposed an improvement of the head-tape contact characteristic by providing a non-magnetic underlayer containing polyisosyanate so that it serves as a cushion.
Further, in the Japanese Patent Laid-Open Publication H4-325917/1992, there is proposed an improvement wherein a thermosetting polyisocyanate is interposed between the non-magnetic underlayer and the magnetic layer. This proposition intends to improve the head-tape contact characteristic by giving a stiffness to the underlayer.
The abovementioned head-tape improvement methods may be effective in such a case as the underlayer is allowed to be coated at a thickness of 3 .mu.m, for instance, in the magnetic tape having a total thickness of 13 .mu.m used in 8 mm VTR. However, the above methods can not be employed in the high density digital recording medium where a further reduction of the thickness thereof (total thickness of 7 .mu.m) is demanded because the thickness of the underlayer has to be reduced inevitably. This poses the problem of insufficient head-tape contact characteristic.
There has not been realized such a magnetic recording medium as satisfies both conditions of the high density digital recording and the extended playing time without increment of the mass of the medium.
Further, the recording signals to be applied are not limited to high frequency signals (short wavelength signals) in the high density digital recording. Such low frequency signals (long wavelength signals) as control signals are also used. Thus, the recording medium is required to have an excellent output level over a wide range from the low frequency signals to the high frequency signals.
Recently, among the recording mediums aimed at reducing the thickness thereof to improve the overwriting characteristic, there are emerged recording mediums employing the magnetic metal powder having an average long axis length of not more than 0.1 .mu.m and a saturation magnetization as of not less than 130 emu/g intended to record the short wavelength of not more than 0.5 .mu.m. However, they do not necessarily display their inherently excellent electro-magnetic conversion characteristics because of difficulty of dispersion of such magnetic particles as highly energized fine particles.